A waste heat steam generator is a heat exchanger which recovers heat from a hot gas flow. Waste heat steam generators are deployed for example in gas and steam turbine power stations in which the hot exhaust gases of one or more gas turbines are conducted into a waste heat steam generator. The steam generated therein is subsequently used to drive a steam turbine. This combination produces electrical energy much more efficiently than a gas or steam turbine on its own.
Waste heat steam generators can be categorized according to a multiplicity of criteria: Based on the flow direction of the gas flow, waste heat steam generators can be classified into vertical and horizontal design types, for example. There are also steam generators having a plurality of pressure stages in which the water-steam mixture contained therein is characterized by a different thermal state in each case.
Generally, steam generators may be implemented as gravity circulation, forced circulation or once-through (continuous) steam generators. In a once-through steam generator, evaporator tubes are heated, resulting in complete evaporation of the flow medium in the evaporator tubes in a single pass. Following its evaporation, the flow medium—typically water—is fed to superheater tubes connected downstream of the evaporator tubes, where it is superheated. The position of the evaporation endpoint, i.e. the point of transition from a flow having residual wetness to a pure steam flow, is in this case variable and operating-mode-dependent. During full-load operation of a once-through steam generator of said type the evaporation endpoint is located for example in an end region of the evaporator tubes, such that the superheating of the evaporated flow medium commences already in the evaporator tubes.
In contrast to a gravity circulation or forced circulation steam generator, a once-through steam generator is not subject to any pressure limiting, which means that it can be dimensioned for live steam pressures far in excess of the critical pressure of water (pCri≈bar)—at which water and steam cannot occur simultaneously at any temperature and consequently also no phase separation is possible.
In order to increase the efficiency of the waste heat steam generator, the latter typically includes a feedwater preheater or economizer. This consists of a plurality of economizer heating surfaces which form the final heating surfaces in the flue gas path following a number of evaporator, superheater and reheater heating surfaces. On the flow medium side, the economizer is connected upstream of the evaporator heating surfaces and superheater heating surfaces and uses the residual heat in the exhaust gases to preheat the feedwater. Disposing the said arrangement in the flue gas duct results in the flue gas flowing through the economizer at relatively low temperatures.
During the operation of a waste heat steam generator it is imperative to ensure adequate supercooling of the flow medium at the evaporator inlet (i.e. the temperature of the flow medium should exhibit a sufficient deviation from the saturation temperature). This guarantees on the one hand that only a single-phase flow medium is present in the distribution system of the evaporator and consequently that no phase separation processes of water and steam can occur at the inlets of individual evaporator tubes; on the other hand the presence of a water-steam mixture at the evaporator inlet would make it difficult if not impossible to achieve an optimal control or regulation of the evaporator outlet enthalpy, as a result of which it might no longer be possible in certain situations to control the evaporator outlet temperatures.
For this reason a waste heat steam generator is typically configured in such a way that adequate supercooling is present at the evaporator inlet at full load. However, the supercooling at the evaporator on the medium side can vary to a greater or lesser degree due to physical conditions, especially in the case of transient load processes.
Additional measures are necessary in the lower load range to ensure adequate supercooling is present in spite of these fluctuations. For this purpose a subflow of the flow medium is typically diverted in a bypass line via a corresponding arrangement around one or more economizer heating surfaces and then mixed with the main flow again for example at the inlet of the last economizer. As a result of such a partial redirection of the flow medium past the flue gas duct the overall thermal absorption of the feedwater in the economizer heating surfaces is reduced and it is thereby ensured that adequate supercooling of the flow medium at the evaporator inlet can be achieved even in the lower load range.
In present-day systems the subflow through the economizer bypass line in the corresponding load range is generally set specifically such that a supercooling of e.g. at least 3 K is maintained at the evaporator inlet during stationary operation. For this purpose there is provided at the evaporator inlet a temperature and pressure measuring means with the aid of which the actual supercooling can be determined at any time instant via a difference calculation. A setpoint-actual comparison causes a valve in the economizer bypass line to be actuated if the minimum supercooling limit is undershot. Said valve receives an opening pulse of e.g. 1 s. A new valve position in which the valve then remains for e.g. 30 s is directly linked with said opening pulse via the valve actuating time. In the event that the required minimum supercooling level is still not reached even after said 30 s, the same process is repeated until either the minimum supercooling level is reached or exceeded or else the valve is fully open.
If, in the inverse case, the measured supercooling is for example greater than 6 K, the valve receives a closing pulse of e.g. 1 s. In comparison with the opening, the valve generally remains in the new position for a greater period of time (for example 600 s) before, following a new alignment between setpoint and actual value, the same process is repeated, should the supercooling at the evaporator inlet be still greater than 6 K and the valve not yet fully closed. In this case comparatively large time intervals are selected between the individual actuating pulses in order to avoid steam formation in the economizer.
As has become apparent, however, in particular in the case of rapid reductions in load, such as occur repeatedly in today's gas and steam installations, it is difficult or even impossible under certain conditions for the above-described control concept to guarantee the required minimum supercooling of the fluid at the evaporator inlet. In the event of said rapid load variations, steam formation at the evaporator inlet could consequently not be ruled out, with the result that problems may occur during the distribution to the individual evaporator tubes and under certain conditions it would no longer be possible to regulate the evaporator outlet temperature.